High Energy Physics - Phenomenology

Title:LO vs NLO comparisons for Z + jets: MC as a tool for background determination for NP searches at LHC

Abstract: The leptonic decays of the heavy gauge bosons W and/or Z provide a clear
experimental signature at hadron colliders. The production of accompanying jets
is an excellent signal to probe QCD, while also being the main background to
many searches for new physics. Describing the complex final state of W or Z +
jets is a theoretical challenge with most existing calculations combining
matrix elements for high energy jet production with a parton shower for lower
energy jet production. We focus on two models: SHERPA, which uses Leading Order
matrix elements for boson and jet production; and POWHEG with HERWIG++, which
uses a Next-To-Leading Order Matrix element for Z production. In order to
isolate the impact of the matrix elements for jet production, it is first
essential to constrain the differences in the rest of the calculation in each
case: specifically, the Multiple Parton Interaction models, and the tuning of
the parton shower interfaced to the matrix elements. We test all three aspects
of these models against data from the Tevatron, and perform a study of some
basic kinematic variables at the LHC energy.